Urethanes have in the past been prepared by the reaction of an aromatic isocyanate with an alcohol, the isocyanate generally having been obtained by the reaction of phosgene with the appropriate primary amine which in turn had generally been obtained by reduction of the corresponding nitro compound. This conventional process has various disadvantages. For example, the toxicity and corrosive nature of phosgene and the formation of hydrogen chloride as by-product have been particularly troublesome. It is also known that certain aromatic amines have harmful biological properties and some of them also tend to be oxidized by air in storage.
There have been many attempts to avoid the use of the highly toxic phosgene and to obtain urethanes directly from the corresponding nitro compounds and the corresponding alcohols and carbon monoxide. The processes according to U.S. Pat. No. 3,993,685 and German Offenlegungsschrift No. 2,603,574 used catalyst systems based on metals of the platinum group. Since considerable losses of the precious catalysts were unavoidable in these processes, they have not so far become established for production on an industrial scale.
In the process described in German Offenlegungsschrift No. 2,343,826, a combination of selenium or sulphur or compounds of these elements with very large quantities of a base was proposed as catalytically active system. The bases used included, for example, triethylamine and pyridine. In order to be able to start the reaction satisfactorily in the presence of these tertiary amines, it was necessary to use them in rather large quantities compared with the nitro compound used as starting material. In fact, when dinitrotoluene is used as the nitro compound, the quantity of tertiary amine used was equal to or greater than that of dinitrotoluene. The use of such large quantities of tertiary amine involves numerous problems of an ecomonic nature and, particularly with regard to the recovery process. Furthermore, this method leads to the formation of by-products such as amino compounds and ureas if measurable quantities of water are present, e.g. as hydrates or in the free form. The process according to German Offenlegungsschrift No. 2,343,826 was therefore unsuitable for application on a large commercial scale.
The reduction in yield of the desired urethanes due to the aforesaid formation of by-products is described as being avoided in the process according to German Offenlegungsschrift No. 2,614,101 by using a catalyst system which is composed of elementary selenium or a selenium compound and a promoter consisting e.g. of a bicyclic amidine and a carboxylic acid. Although the process according to German Offenlegungsschrift No. 2,614,101 resulted in higher yields of urethanes than the process according to German Offenlegungsschrift No. 2,343,826, it also led to disturbing quantities of by-products, particularly products of hydrolysis and of secondary reactions of the urethane formed.
The process according to German Offenlegungsschrift No. 2,623,694 is regarded as a further development of the process described in German Offenlegungsschrift No. 2,614,101 in that the formation of by-products from urethanes by the addition of aromatic amino compounds or aromatic urea compounds which correspond to these by-products was suppressed. Although this measure provided an improvement to the process of German Offenlegungsschrift No. 2,614,101, the process of Offenlegungsschrift No. 2,623,694 still had serious disadvantages. In particular, it required the use of exceptionally large quantities of selenium or selenium compounds, so that considerable losses of this catalyst necessarily occurred. Furthermore, selenium and the selenium compounds used as catalysts are toxicologically harmful substances apart from imparting an unpleasant odor to the urethane produced.
It was therefore an object of the present invention to provide an improved process for the production of urethanes from aromatic nitro compounds, alcohols and carbon monoxide in which the quantity of selenium or selenium compound could be very substantially reduced and urethane could be formed substantially quantitatively in spite of the reduction in the quantity of catalyst.